This may seem a bit redundant, but I would like to second the recommendation of the NuVinci for any cargo bike. I am a huge fan of internally geared hubs in general, and have ridden them on many bikes, and worked on many different hubs over the years. My impression of any of the "normal" geared hubs (Nexus, SRAM, or Sturmey) is that they work beautifully on normal bikes, but are not really durable enough for use with anything that will take a payload of more than 100 lbs. The SRAM hubs seem to be the most durable of them all, but even they can be destroyed by heavy loads, steep hills and agressive riding (it takes a while, but I've done it). The Rohloff, of course, is in a class by itself, and no one has been able to destroy one of them (that the maker knows of). If you can afford it, there is not much to be said against it; the only possible drawback is that it still has discrete shift points, and needs to disengage from one gear and engage the next before continuing to transmit torque - what we've always been used to, in other words. The NuVinci is particularly suited to heavy use like this, since it has no actual gears inside to wear down or lose teeth. The continuous nature of the shifting mechanism means that it is always in gear, and therefore can never be mis shifted. It is also extremely easy to downshift in overloaded situations; any time the pedal is unweighted a bit, the hub can shift, so you get just a bit of shift at every dead spot in your stroke, or a big shift, if you want, if you actually pause intentionally. I've been trying to overload mine, just to see, and so far the hub is much stronger than I am, which is how it should be. Just for reference, I am also using it with two chainrings, and it gives me a range of 20 gear inches to 90 gear inches - enough, in other words. For the price, nothing can touch it. For another perspective on a NuVinci Dummy, see:
http://tinyurl.com/4743tl